9.29.20 Lesson #5 Small Intestine: Duodenum
After all that work in the stomach, you’d think digestion would be pretty basic after leaving the stomach, right? Not really! Welcome to the first 10 inches of your small intestine!
Chyme arrives in the duodenum about 2-4 hours after you’ve eaten. A bunch of things have to happen here! The acidity of the chyme triggers the production of mucus containing sodium bicarbonate that alkalizes the chyme so the acid doesn’t burn up the more sensitive membranes. (Only the stomach can handle that level of acidity!) The cells that produce the mucus are called goblet cells. (Would you like a goblet of mucus?!) There are two hormones also being secreted; secretin and cholecystokinin (CCK). CCK triggers pancreatic juice (enzymes), contracts the gallbladder to squeeze out the stored bile, and contracts the pyloric sphincter (between the stomach and duodenum) which slows your stomach from emptying and makes you feel full, satiety. Secretin is busy stimulating that bicarbonate to lower the pH.
By the time chyme leaves the duodenum it is almost completely digested (NTA slides 2019). It has broken down carbohydrates, proteins, and fats into glucose molecules, amino acids and polypeptides, and glycerol molecules respectively. These must be broken down this much by this point to be absorbed and utilized in the rest of the small intestine. The chyme mixed with bile and all the juices from the accessory organs is now moved by peristalsis to the next part of the small intestine. Peristalsis is the muscles in a tube constricting and relaxing to move an object down its path (think of putting a rubber band around a garden hose).